Electrophotographic process and apparatus involving persistent internal polarization

ABSTRACT

A PROCESS AND APPARATUS FOR FORMING AN ELECTROSTATIC IMAGE IN A PLATE HAVING AN INSULATIVE LAYER, A PHOTOCONDUCTIVE LAYER UNDERLYING SAID INSULATIVE LAYER AND A BASE, WHEREIN THE INSULATIVE LAYER IS INITIALLY CHARGED IN ONE POLARITY AND IS THEN SUBJECTED TO A.C. CORONA DISCHARGE   WHILE THE PHOTOCONDUCTIVE LAYER IS EXPOSED TO AN IMAGE PATTERN. THE PHOTOCONDUCTIVE LAYER IS THEREAFER EXPOSED TO ACTIVATING LIGHT WHEREBY AN INTENSIFIED ELECTROSTATIC IMAGE IS FORMED.

May 30, 1972 HIROSHI TANAKA ETA!- 3,666,355

ELECTROPHOTOGRAPHIC PROCESS AND APPARATUS INVOLVING PERSISTENT INTERNAL POLARIZATION Original Filed Aug. 10, 1966 ea 0 s0 3 IVENTORS fi wosm Film/m 18 By 007/ 7564/04 5/0/0000 MAW/Maw States Patent once 3,666,365 Patented May 30, 1972 U.S. Cl. 355-17 14 Claims ABSTRACT OF THE DISCLOSURE A process and apparatus for forming an electrostatic image in a plate having an insulative layer, a photoconductive layer underlying said insulative layer and a base, wherein the insulative layer is initially charged in one polarity and is then subjected to AC. corona discharge while the photoconductive layer is exposed to an image pattern. The photoconductive layer is thereafter exposed to activating light whereby an intensified electrostatic image is formed. 7

This application is a division of United States application Ser. No. 571,538, filed Aug. 10, 1966, now abandoned.

The present invention relates to electrophotography in general, and in particular, this invention relates to methods and apparatus for forming electrostatic and electrophoto graphic images. v

Known electrophotographic methods include such conventional methods as the Electro Fax system, the Xerox system, the P.I.P. (Persistent Internal Polarization) 'sys tem and the like. The 'Electro Fax and Xerox'systems form electrostatic images by means of the so-called Carlson process 'as described in the specification of US. Pat. No. 2,297,691. According to these systems, the photoconductive layer of a photosensitive plate comprised of zinc oxide (Electro Fax), or non-crystalline selenium (Xerox) disposed on a base plate, is uniformly charged by corona discharge, and thereafter is irradiated with an original image to impart charge to the illuminated portion to form an electrostatic image in accordancewith the light-and-dark pattern of the original image. The electro static image is developed by using electroscopic powder (hereinafter called toner) to forma visual image, and

then the Sald Image Is 'fixed (Electro Fax) or transferred I process, an electrostatic image is formed by such electroonto a support such as paper and thereafter it is fixed (Xerox) to obtain an electrophotographic image. In ac cordance with P.I.P. system, the photosensitive plate comprises a mixture of phosphor and resin disposed on a conductive base plate and is pinched with two electrodes. A voltage is applied to the two electrodes to generate persistent internalpolarizing charge in the photoconductive layer, and then by irradiating the plate vwith an original image, an electrostatic image is obtained by persistent internal polarizing charge in accordance with the light-and-dark pattern of the original image. Thereafter development and fixing processes are carried out in the same manner as in the above-mentioned cases, and an electrophotographic image is obtained.

have high resistivity, such as non-crystalline selenium, ZnO-l-resin, ZnCdS-i-resin or the like.

Therefore, machines in present practical use have low sensitivity, and in the case of Electro Fax, the sensitivity is below ASA 5 even if acceleration of sensitivity should be carried out by using dyes, and even in the case of Xerox system, or P.I.P. system, the sensitivity is ASA 10 at maximum.

When the above-mentioned photosensitive plate is repeatedly used scars on the surface, or other deterioration of the surface easily occurs, and the quality of the image deteriorates because of fatigue of the photoconductive material. Thus, such plates cannot withstand repeated use over long periods.

There has been proposed a further method described in the specification of US. 'Pat. No. 3,124,456 according to which a photosensitive plate having a photocon: ductive layer composed of CdS or CdSe and binder resin on a conductive base. is provided with a translucent insulating layer overlaid on the photoconductive layer. Irradiation of the original image and charging are carried out simultaneously from the translucent insulating layerside of the photosensitive plate and the electrostatic image is formed on the translucent insulating layer by makinguse of the difference of the buildup of the charge due to. the difference of time constants caused by the different impedances of the photoconductive layer in the light and dark portions, respectively, of the original image. (in accordance with this method, electrostatic image formation depends on the time constant difference :brought about by the differences of impedance in the photoconductor and therefore the electrostatic contrast is not high. To obtain an excellent image by means of this method, the capacitance of the translucent insulating layer must be larger than the capacitance of the photoconductive layer, and

1 from a practical point of view the thickness of the trans- On the other hand, according to US. Pat. No. 3,041,167

issued to R. M. Blakney et al., a photoconductive layer is provided on a conductive base plate, and the photosensitive plate is obtained by protecting the Surface thereof with an overcoating layer. In accordance with the Carlson coming the fatigue of the photosensitive plate. Photo-carriers produced by the whole surface exposure and having a polarity opposite to that of the charge of the insulat- In the above-mentioned systems, it is necessary to re-' ing layer are induced at the interface between the insu lating layer and the photoconductive layer. Then the sensitizing charge is applied in the dark to the surface ofthe insulating layer to neutralize the charge to form a surface field due to the induced charged layer. Thus the photosensitive plate is sensitized. Next, the light image is exposed to attenuate said induced charge and the latent image is formed by the charge remaining at the dark areas. This process is a Carlson-type process and the obtained contrast is as much as 300 to 500 volts.

In the photosensitive plate of the above-mentioned process," it is necessary that the overcoating layer" be" thin compared to the photoconductive layer. Therefore it is easy to bring about wear or breakdown or such like troubles and it is impossible to sufficiently protect the photoconductive layer.

In accordance with the present invention, quite different from the above mentioned process, a photosensitive plate exhibiting internal polarization characteristics and comprising a base, a photoconductive layer overlying said base and an insulative layer overlying said photoconductive layer, is first polarized in one'direction by the application of a charge of one polarity onto the insulative layer, and then while exposing the photoconductive layer to a pattern of image light, alternating current corona discharge is applied to the insulative layer, to obtain a surface potential difference on the insulating layer in accordance with the light and dark pattern of the image light. The photoconductive layer is thereafter exposed to activating light whereby an intensified electrostatic image is formed. An object of the present invention is to provide a new electrophotographic process and apparatus which can be repeatedly used for a long period of time by overcoming the drawbacks of the above-mentioned conventional electrophotographic methods.

Another object of the present invention is to provide an electrostatic image forming process comprising charging in positive or in negative, in advance, the surface of the translucent insulating layer of a photosensitive plate comprising a photoconductive layer overlaid on an insulating base body, said translucent insulating layer being overlaid on the said photoconductive layer, by means of electrodes or corona discharge or the like; and then irradiating the original image on the said insulating layer, and during the same time applying alternating current corona discharge thereto, and thus forming an electrostatic image defined by the surface potential generated in accordance with the light-and-dark pattern of the original image on the said translucent insulating layer.

A further object of this invention is to provide an electrostatic image-forming process comprising forming a first electrostatic image as mentioned above, and thereafter irradiating light all over the translucent insulating layer, and thereby form a second electrostatic image of the original image of high contrast on the surface of the said insulating layer.

Another object of the present invention is to provide an electrophotographic process which comprises visualizing the electrostatic image formed as mentioned above by using developer, transferring the thus obtained visualized image onto a transfer material, fixing the same to obtain an electrophotographic image of the original image, and repeatedly using the photosensitive plate'by cleaning the surface of the insulating layer after theelectrostatic image is transferred.

Another object of the present invention is to provide an electrophotographic image forming process comprising visualizing the electrostatic image formed on the translucent insulating layer as mentioned above by using electroscopic developer, and then charging the translucent insulating layer containing the said visualized image with optional polarity, and overlaying a transferring material thereon -to transfer the visualized image, fixing the trans ferred image, for example, by means of heat, to obtain an electrophotographic image, and after having carried out the transfer of the image, cleaning the surface of the said insulating layer to remove the remaining developer, and using the said photosensitive plate over and over again. I

The above-mentioned objects and other numerous objects of therpresent invention, and a number of characteristics and effects of this invention will be easily and clearly understood from the explanations of the embodiments of this invention shown in the drawings, in

which:

FIG. 1 is a diagram showing the fundamental structure of an electrophotographic plate to be used in the process for forming the electrostatic image of the present invention; I r

FIGS. 2 through 5 show charge patterns of a photosensitive plate and a process for forming an electrostatic image on the translucent insulating. layer'of the plate; and i FIG. 6 is a diagram explaining how to obtain negativepositive images by means of toner.

FIG. 1 is a diagram which shows the fundamental structure of an electrophotographic plate used in the process for forming the electrostatic image of the present invention, and in the diagram, 1 isa base, 2 is a photoconductive layer coated on the base by using a sprayer, or a coater, or wheeler or the like, and if necessary, it is possible to add a little amount of binder material such as resins and the like. 3 is an insulating (insulative) layer which is closely adhered on photoconductive layer 2. Thus, photosensitive plate A' has three layers, i.e., base 1, photo: conductive layer 2, and insulating layer 3. It is also possible to form a control layer such asto control the transfer of charge between base 1 and photoconductive layer 2, and it is also possible to add or independently provide a layer for catching charge on the surface of photoconductive layer or in the neighborhood of the surface.

Base 1 is formed of insulating material-As an insulating base, it is possible to use the same material which constitutes the insulating layer 3, as discussed hereinafter, but it is not necessarily the same material. It is possible to freely select any optional insulating materials, and by coloring insulating layer 3, it is also possible to obtain antihalation quality as is generally known in photography.

As the material which composes photoconductive layer 2, CdS, CdSe, metallic Se, ZnO, ZnS, Se, TiO Se-Te, PbO and S or like inorganic photoconductors, or anthracene, carbazole, or like organic photoconductors can be used, and it is also possibleto use any of the above-men: tioned materials by directly coating same on the base, or in the form of mixture along with binder, or a mixture of more than two kinds of the above mentioned materials.

Among these photoconductive materials, highly sensitive materials especially adapted for the present invention, are highly photoconductive materials such as CdS, CdSe, metallic selenium, Se-Te, orthe like, and when these ma: terials are used, it is possible toelevate sensitivity up to ASA 100. On the other hand, a photoconductive layer obtained by adding a little amount of ZnS in a photo: sensitive layer mainly composed of CdS, is highly sensitive, and provides an electrostatic image of high contrast and high sensitivity.

Mixtures of CdS and ZnS have hitherto been used (in P.I.P. system), but in order to elevate the difference of photo-polarization and dark polarization and internal polarization characteristics, the ratio of CdS to ZnS is selected to be within the range of 4:6 to 3 :7.

On the contrary, in the case of the present invention, the ratio of CdS and ZnS is preferably within the range of 50:1 to 1:1, and it is possible to make advantageous use of the characteristics of the highly sensitive CdS.

Photoconductive papers incorporating zinc oxide dispersed in a resin, which have hitherto been adopted in the conventional Electro Fax system, are required to be white because they are used as copying papers by themselves, and therefore it isimpossible to add much dye to sensitize same, and it has not been possible to elevate the sensitivity sufiiciently.

However, in accordancewith the method of the pres.- ent invention, the photosensitive plate is itself not used as copying paper, but avisualized image is made on transfer paper, and therefore it is not necessary for the photosensitive plate to be white, which enables one to add remarkably larger amount of dyes when compared with th amount of dyes used in conventional methods.

' Therefore, it is possible to use, in accordance with the method of the present invention, a several times more sensitive zinc oxide photoconductive layer compared with conventional method. 7 1

In the method of the present invention, a photoconductive layer obtained by doping lithium into ZnO presents excellent results; v 2

As to the material which constitutes insulating layer 3, any material can be used which has high resistance against wear, high resistivity and capability of binding electrostatic charge, and translucency to activating radiation. Films of the following resins can be used, i.e., fluorine resin, polycarbonate resin, polyethylene resin, cellulose acetate resin, polyester resin, or thelike. In particular, fluorine resin has a specific property which makes, it easy to carry out cleaning, and therefore as is explained hereinafter, it is a preferable material in carrying out the method of the present invention for using the photoconductive plate over and over again through developing, transferring, and cleaning processes.

The processes for, forming an electrostatic image on the translucent insulating layer of a photosensitive plate having a base formed of insulating material will now be discussed. I

FIGS. 2 through 5 show the charge patterns of photosensitive plate A and processes for forming an electrostatic image on translucent insulating layer 3 thereof. By the first or primary charge process step (FIG. 2), and the process step for carrying out original image irradiation while applying alternating current corona discharge (FIG. 3), the-electrostatic image shown in FIG. 4 is formed on the surface of the translucent insulating layer. As further radiation is applied onto the surface of the insulating layer, the electrostatic image shown in FIG. 5 is formed on the surface of insulating layer 3. w

To prepare photosensitive plate A, photoconductive layer 2 is overlaid on insulating layer 3 and translucent insulating layer 3 isoverlaid onphotoconductive layer 2. On translucent insulating layer 3 and insulating layer 3', corona dischargesof opposite polarities are applied by means of direct current corona dischargers and 16 connected respectively to the and terminals of high voltage electric source 14. Insulating layers 3 and. 3' are thus charged with opposite polarity, respectively. In order that the polarized charges may .be iincreased, and that the hysteresis of the photoconductive materials can be erased, the whole surface of the photosensitive plate can be radiated, at the same time, by-tungsten light source 17, orthelike. a i I- In this primary charge step, as is shown in FIG. 3, instead of applying corona discharge on the surface of insulating layer 3', the photosensitive plate can be placed on a conductive base plate 18 which is'grounded. Although it is not shown in the drawing, conductive base plate 18 can be connected to a direct current source of polarity opposite thatof corona discharger 15.

Now, in order to have the explanation clearly understood, the primary charge is presumed to be in the positive.

Of course, in the present embodiment, when both sides of photoconductive layer 2 are overlaid with insulating layers, regardless of the properties of photoconductive material, charge polarity can optionally be selected.

After the completion of the first charge process, internal polarization is generated in photoconductive layer 2 of the photosensitive plate as shown in FIG. 2.

Next, as shown in FIG. 3, alternating current corona discharge is carried out while irradiating the original image onto charged translucent insulating layer 3 through alternating current corona discharger 20 whose upper portion is optically open, and which is connected to alternating current high voltage electric source 19. In FIG. 3, an embodiment is shown where the insulating layer 3' is placed on a grounded conductive base plate. Where alternating corona discharge is carried out along with irradia tion of the original image, the charge on the surface of the insulating layer is released at the light areas a of the original image as shown in FIG. 4, and internal polarizing charge is released, and at the same time, at the dark area b. the internal polarizing charge of the photoconductive layer is not substantially released, and therefore the charge of the insulating layer is not wholly but partially eliminated. Therefore, electrostatic contrast can be obtained between the light areas of the original image and the dark area of the same.

Next, as is shown in FIG. 5, when light is irradiated uniformly on the whole surface of the insulating layer 3, the state of the photoconductive layer is not changed much at the light areas a of the original image, but at the dark area b thereof, the photoconductive layer is rendered conductive and therefore the polarizing charge therein which was inactive in the preceding process step, is activated, and released and eliminated. Therefore, the external field on the surface of the insulating layer, is increased, and the surface potential is quickly increased, to a value higher than that of the surface potential of the light area, and the difference therebetween is increased, providing an electrostatic image of high contrast on the surface of the insulating layer.

The electrostatic image formed as mentioned above becomes an electrophotographic image through the abovementioned process steps of development, transferring and fixing and it is possible to repeatedly use the photosensitive plate after transferring the electrostatic image by cleaning same.

- When the electrostatic image is visualized by means of toner charged in the positive as shown in FIG. 6, a negative to positive image can be obtained, while when the imageis made visible by using toner charged in the negative, a positive to positive image can be obtained.

0n the other hand, as is apparent from the above explanation, the present invention makes use of photoconductivity and at the same time makes use of the difference of electrical fields of photopolarization charge, and thereby a different charged state can be obtained by means of alternating current corona discharge. Thus, by changing ,the primary charge polarity it is possible to obtain either a negative to positive image or a positive to postive image by using one and the same developer. This is greatly different from conventional electrophotographic methods according to which charging polarity is selected depending on the n-type or p-type of photo conductor.

Also, as is shown in FIG. 2, by applying double corona discharges to the insulating layers provided on both surfaces of the photoconductive layer, a positive image can be obtained on onesurface, and a negative image can be obtained on the reverse surface. The photosensitive plate is composed by coating .both sides of photoconductive layer 2 with highly insulating layers 3, 3, and it is possible to form a flexible photosensitive plate in belt form and, since both sides are coated with highly insulating layers, deterioration of the photoconductive substance as is caused by the absorption of humidity can be prevented. Thus, in accordance with the present invention, a remarkable effect which is much greater than the effect possessed by photosensitive plate A, can be attained.

In the electrostatic image forming process of the present invention, the thickness of the translucent insulating layer 3 affects the quality of the electrostatic image along with the photoconductive layer. In particular, it affects sensitivity, contrast, and durability of the photosensitive plate, which are important factors, and in order to form an excellent electrostatic image and in order to use the photosensitive plate repeatedly for a long period of time, it is necessary that the thickness of the translucent insulating layer be within the range of 10 to 50p.

Other photosensitive plates'of the present invention will be explained hereinafter. Photosensitive plate A is obtained by overlaying photoconductive layer 2 on insulating layer 3, and overlaying translucent insulating layer 3 on layer 2. Insulating layer 3"'is formed in a thickness from 10 to 50p. and may be formed of the same material as the translucent insulating layer 3, and in such case, it is possible to alternate use of both sides of'the photosensitive plate as the electrostatic image-forming surface, resulting in advantageous increase in durability.

We claim:

-1. A process for forming an electrostatic image in a photosensitive plate having a base, a photoconductive layer overlying said base and an insulative layer overlying said photoconductive layer, said photosensitive plate exhibiting internal polarization characteristics, comprising the steps of: i

(1) applying a charge of one polarity to said insulative layer to polarize said photosensitive plate in one direction, then (a) while exposing said photoconductive layer to a pattern of image light, y (b) applying an alternating current discharge to said insulative layer; and then i (3) exposing said photoconductive layerwith activating light to release said polarization in said one direction and form a high contrast electrostatic image.

2. The process claimed in claim 1' whereinsaidphotoconductive layer is exposed to said pattern of image radiation through said insulative layer. I

3. The process claimed in claim 1 wherein said photoconductive layer is exposed to said activating light through said insulative layer.

4. The process claimed in claim 1 wherein said step of applying said charge of one polarity to said insulative layer is performed while exposing said photoconductive layer to activating light.

5. A process for forming an electrophotographic image comprising the electrostatic image-forming process claimed in claim 4 and the further terminal steps of:

(4) applying a developer to said plate to visualize the electrostatic image latent in said plate;

(5) transferring said visualized image onto a transfer member; and

(6) fixing said visual image on said transfer member.

6. A process for forming an electrophotographic image comprising the electrostatic image-forming process claimed in claim 1 and the further terminal steps of:

(4) applying a developer to said plate to visualize the electrostatic image latent in said plate;

(5) transferring said visualized image onto a transfer member; and

(6) fixing said visual image on said transfermember.

7. The process claimed in claim 1 wherein said photoconductive layer is exposed to said pattern of image radiation through a corona discharger applying said alternating current discharge to said insulative layer.

8. The process claimed in claim 1 wherein said plate includes a conductive member underlying said insulative base.

9. The process claimed in claim 1 wherein said base is of an insulative material. 1

"110. An apparatus for forming. an electrostatic image comprising:

(a) a photosensitive plate having a base, a photoconductive layer. overlying said base and an insulative layer overlying said photoconductive layer, said photosensitive plate exhibiting internal polarization 1 characteristics, a r

(b) means for applyingwa charge of one polarity, to said insulative layer to polarize said photosensitive plate in one direction, Y 1 l (0) means for exposing said photoconductive layer to a pattern of image light while'applying an alternating current corona discharge onto said insulative layeryand (d) means for thereafter exposing said photoconductivelayer to activating light to release said polarization in said one directionand form an'electrostatic image. r w

11. An apparatus according to claim 10,'further comprising: a a r (e) means for uniformly exposing said photoconduc tive layer simultaneously during the applicationof said charge of one polarity to erase the hysteresis of said photoconductive layer. a I

12. An apparatus according to claim 10, wherein said base is insulative material.

13. An apparatus according to claim 10, wherein said covering insulating layer is transparent both to said original image and activating light, and said means (c) and (d) include means for exposing said photoconductive layer through said transparent insulating layer.

14. An apparatus for forming an electrophotographic image comprising the electrostatic image-forming apparatus claimed in claim 10 and further comprising:

' (f) means for visualizing the electrostatic image,

-(g)' means for transferring the visualized. image onto a transfer sheet,

" (b) means for fixing the visualized'image, and

' (i) means for cleaning the surface of the insulating layer'to remove residual developer and enable the repetitive use-of said photosensitive plates.

References Cited UNITED STATES PATENTS 3,199,086 8/1965 Kallmann et al. 355.l 7 X 3,084,061 4/1963 Hall 355-47 X 3,457,070 7/ 19,69 Watanabe (it al. -961 RX 3,536,483

' 10/1970 Watanabe et a1. 96- 1 R JOHN M. HORAN, Prim... Examiner K. C. HUTCHlSON, Assistant Examiner 

